fix(delegation): honor worker fallback providers

[wz-heng]

Summary

• honor delegation.fallback_providers when constructing delegated child agents
• preserve parent-chain inheritance when the delegation key is absent
• treat an explicit empty delegation chain as disabling inherited fallbacks
• normalize worker fallback entries through the shared fallback-chain helper

Fixes #65038

Validation

• python -m pytest tests/tools/test_delegate.py tests/run_agent/test_provider_fallback.py -q (79 passed)
• ruff check tools/delegate_tool.py tests/tools/test_delegate.py
• git diff --check origin/main...HEAD

[alt-glitch]

This was generated by AI during triage.

Duplicate of #65052. Both implement the same worker fallback-chain precedence and null/empty inheritance semantics for #65038; #65052 is the earlier, broader open implementation.

[wz-heng]

Closing as a duplicate of #80479, which covers the delegation fallback behavior with the full pin × config matrix, defaults, and documentation.